This invention relates to optical transmission cables in general and more particularly to a cable which includes provisions for improved protection against mechanical stress to the transmission elements in the cables.
In the past few years there has been a great deal of development in the area of optical cables for the transmission of light waves. Such optical cables are being used, for example, for the transmission of control pulses or more generally for transmission of information in the field of communications engineering. Essential components of such cables are glass fibers which are used to guide the light beam carrying the desired information. In designing such optical cables, the mechanical properties of the glass fibers must be taken into consideration. In other words, the glass fibers must be protected, particularly against mechanical stresses such as compression, tensile stresses and being stresses. In one known optical cable, several optical transmission elements in the form of glass fibers are disposed together in at least one stranded layer about an elongated central core which is used to take up tensile stresses. Both under and over the stranded layer, a cushion layer of polyethylene foam is arranged. Over this there is disposed an open covering of copper tape and an outer sheath of plastic such as polyethylene or polyvinylchloride. Such is disclosed, for example, in British Pat. No. 1,425,928.
In another known optical cable, several glass fibers are combined in a bundle and a plurality of bundles twisted together. Between the bundles one or more stress relief wires are disposed and the spaces between the individual glass fibers and between bundles filled with a lubricant. The bundles which are twisted together are surrounded by a first protective sheath on which a metal foil is arranged which in turn is surrounded by a second protective sheath.
The known sheath designs for optical cables are relatively resilient and do not insure sufficient protection of the optical transmissions elements particularly if the optical transmission elements are coupled directly to the cable sheath due to the cable design used. Thus, in some cases, under certain types of mechanical stress, damage to the cables can occur.
In view of this the need for an improved arrangement for protecting optical transmission elements in an optical cable becomes evident.